is my zebra finch okay?

Question
I am trying to figure out if my recently adopted zebra finch is okay. About 3 weeks go I adopted 4 male zebra finches from an avian rescue organization. One does not act like the others. I have no previous experience with birds and am unsure how concerned I should be. He is more sedentary than the others. He often sits in his nest during the day for periods of time. He will be sedentary on a perch for a while without flitting around the way the others do, and he often sits with his body down over his legs, like a hen sitting on a nest, while the others tend to stand on their perches. He sometimes stands on one leg and lifts the other up into his feathers. And he is much shyer than the other birds. On the other hand, he is alert, he does eat, he does fly around, just nowhere near as much as the others. I think maybe he acts the way he does because he is favoring on leg, trying to to use it much or put weight on it. If that is what is going on, should I take him to a vet or might it get better on its own?

I don't know if this could be related, but when I first got the finches home, I noticed that this bird was missing some feathers behind his head. The rescue agency said he came in that way and to watch it to make sure it didn't get any worse. It did get worse initially, for the first week, but does not seem to be any more. I got them a much larger cage and put some more interesting things in it to occupy them, which seems to have made them all happier in general and I am hoping that this has helped discourage his partner from plucking him. So, while I think that the plucking has stopped, the feathers have not grown back yet.  

Answer
It sounds like he is being picked. Best to watch the cage and separate the trouble maker. Things should get better. I wouldn't worry too much about the leg thing. Birds will favor one leg over the other when they are resting. It's when they're wide awake and favoring a foot that you need to worry. If things don't improve after you have found your trouble maker in about a month, take your little baldy to the vet